Japanese leaders did not know the extent of the damage in the wake of the nuclear crisis after the massive quake and tsunami hit the Pacific nation even as they tried to play down the risk in public, an independent investigation is set to report.

Naoto Kan, the then-prime minister, and his staff began referring to a worst case scenario that could threaten Japan's existence as a nation around three days after the quake-triggered tsunami on March 11, and even secretly considered evacuating the capital, Tokyo, the report by the Rebuild Japan Initiative Foundation said on Tuesday.

Kan ordered workers to remain at the tsunami-crippled Fukushima nuclear plant as fears mounted of a "devil's chain reaction" that would force tens of millions of people to flee Tokyo, the report says.

That was when fears mounted that thousands of spent fuel rods stored at a damaged reactor would melt and spew radiation after a hydrogen explosion at an adjacent reactor building, according to the report.

Spectre of crisis

In an interview with Reuters this month, the 65-year-old Kan said he was haunted by the spectre of a crisis spinning out of control and forcing the evacuation of the Tokyo greater metropolitan area, 240km away and home to some 35 million people.

Yukio Edano, then Japan's top government spokesman, told the think-tank that at the height of tension he feared a "devil's chain reaction" in which the Fukushima Daiichi plant and the nearby Fukushima Daiichi facility, as well as the Tokai nuclear plant, spiralled out of control, putting Tokyo at risk.

"I had this demonic scenario in my head" that nuclear reactors could break down one after another, he is quoted as saying."If that happens Tokyo will be finished.

A team of 30 university professors, lawyers and journalists investigated Japan’s response to the triple meltdown at the Fukushima plant, which followed the twin disasters that shut down the plant’s cooling systems.

Since September, it has interviewed more than 300 people, including Kan, then-trade minister Banri Kaieda and Edano.

Lack of trust

The New York Times newspaper, which had an advance copy of the report, said there was lack of trust between the major actors: Kan; the Tokyo headquarters of the plant’s operator, Tokyo Electric Power (TEPCO), and the manager at the stricken plant.

Kan's administration, TEPCO and nuclear regulators have all faced criticism, both for a confused response and for failing to come clean on the extent of the crisis in the early days, undermining public trust in Japan's leaders and bureaucracy.

Imad Khadduri, a nuclear expert, agreed that the clean-up effort will take a long time.

“It will take 30 to 40 years to clean the plants that are destroyed, to remove the radioactive fuel, the spent fuel, and to entomb everything," he said.

"To decontaminate the ocean or the soil will also take a couple of decades. Even now in Chernobyl [site of the 1986 nuclear disaster in the former Soviet Union], they haven’t finished after 30 years."